[NUI] Interops for accessibility (#2277)

[NUI] Interops for accessibility (#2277)

* [NUI][AT-SPI] Add accessiblity support to View

This commits add support for 2 signals:
 * generic Signal<void(string)>
 * atspi custom Signal<void(std::pair<GestureInfo,bool>)>
Add accessibilty properties to View.
Add accessibilty events to View.

Change-Id: I795bbb794a0f0c96210ddf4ad387386c1e1f8777

* [NUI][AT-SPI] Add AccessibilityDelegate

* [NUI][AT-SPI] Add basic Accessibility methods

* Initial implementation of the method which sets the Accessibility
states of a View
* Initial implementation of the overriden method for Button class

* [NUI][AT-SPI] Add more AccessibilityDelegate methods

* [NUI][AT-SPI] Added 2-finger pan possibility to ScrollableBase

* [NUI][AT-SPI] Basic Accessibility for Slider

* [NUI][AT-SPI] Basic accessibility for Dialog

* [NUI][AT-SPI] Clean up code

Rename structures with event arguments.
Hide at-spi implementation.

Change-Id: Icf578f977dc096fc0813c02aa8db2d5434c4a0f0

Co-authored-by: Artur Świgoń <a.swigon@samsung.com>
Co-authored-by: Maria Bialota <m.bialota@samsung.com>